1. Field of the Invention
The present invention relates to systems for conducting a pre-treatment, i.e., a pre-linearization and a pre-distortion, of an input signal to be transmitted via a non-linear amplifier, in particular a power amplifier, in order to compensate for level-dependent non-linearities of the gain of the amplifier.
The present invention can be used for conducting a pre-treatment of input signals formed using modulation methods which result in a non-constant envelope of the radio frequency carrier signal. Thus, the present invention preferably finds use in transmitters for digital broadcasting, which are fed by multi-carrier signals, such as OFDM signals (OFDM=orthogonal frequency division multiplex), for example. With such signals, the non-linearity of the amplifier causes unwanted frequency portions of the signal at the output of the amplifier. Such frequency portions interfere with adjacent-frequency channels.
In addition, the present invention is applicable to mobile communication systems using CDMA signals (CDMA=code division multiple access), for example. Additionally, the present invention can be advantageously used in satellite earth stations or base stations of mobile telephone systems.
When pulse amplitude modulation methods are used, the spectral efficiency will deteriorate due to the non-linearity of the amplifiers. The reason for this is to be seen in the non-linear amplitude output characteristics of an amplifier, which leads to an AM/AM conversion; the phase drift of an output signal of an amplifier relative to the phase of an input signal produces, in addition, intermodulation components, which is also referred to as AM/PM conversion. The AM/AM and the AM/PM conversion must, however, be prevented by suitable linearization methods. If this is not done, the spectral efficiency of the modulation method used as well as the signal-to-noise ratio will deteriorate. In digital transmission systems this can cause a substantial increase in the bit error rate during the transmission.
2. Description of Prior Art
A plurality of methods for linearizing radio frequency final stages in order to reduce spurious adjacent-channel emission at the output thereof already exist in the field of technology. The best-known methods of linearizing radio frequency final stages can be classified as follows.
When a signal to be transmitted is digitally pre-distorted, the digitally represented values of the signal are multiplied by suitably selected coefficients. Hence, the pre-distortion is carried out together with the digital generation of the control signal of the modulator.
A further known method is the analog pre-distortion. It makes use of non-linear components, such as Schottky diodes, so as to synthesize an equalizing characteristic which is complementary to the amplifier distortion characteristic.
The xe2x80x9cCartesian loopxe2x80x9d represents an analog negative feedback of the radio frequency final stage, which is carried out in the baseband.
The forward coupling (also referred to as xe2x80x9cfeedforwardxe2x80x9d in the field of technology) constitutes a disturbance variable feedforward in the sense of control technology, the output signal of the final stage having added thereto a suitable correction voltage for compensating the distortion of the final stage.
WO 93/18581 describes a xe2x80x9ccartesian loopxe2x80x9d whose parameters are adjusted in accordance with various system parameters which are representative of the current operating condition of the system. A radio transmitting unit according to said WO 93/18581 comprises a power amplifier, a linearization means and a feedback means for feeding a signal back from an output of the power amplifier to the linearization means so as to guarantee the linearity of the output signal. The linearization means operates in the baseband, the IQ signals being controlled by a linear control means which is connected to a direct-access table storing predetermined loop linearization parameters. When the IQ signals have been processed in a suitable manner by the linearization means, these processed signals are subjected to up-conversion by an up-conversion mixer whereupon they are amplified by the power amplifier. The feedback means takes an output signal of the power amplifier, subjects it to down-conversion by means of a down-conversion mixer and feeds the down-converted signal into the linearization means.
GB 2240893 A discloses a circuit for linearizing the amplitude response and the phase response of an amplifier. An envelope detection circuit detects the envelope of an input signal to be transmitted, the output signal of said envelope detection circuit being inputted in a control circuit of the non-linear type as well as in a phase shift control circuit. The phase shift control circuit controls a phase shifter which precedes the power amplifier for pre-distorting the radio frequency signal with regard to its phase. The control circuit of the non-linear type feeds an input signal into a variable-voltage dc-dc converter which adjusts the bias voltage parameters, i.e. the operating point of the power amplifier in a suitable manner so as to compensate the distortion of the non-linear amplifier. The amplitude error of the amplifier is therefore compensated for by adjusting the operating point of said amplifier, a course of action which is disadvantageous insofar as the operating point parameters of the amplifier will have to be changed constantly, and this may it make much more difficult to adapt the amplifier to a load, since, normally, a changed operating point will automatically require a different (complex) transformation ratio of the output resistance.
U.S. Pat. No. 5,023,937 represents an analog pre-distortion circuit for a power amplifier operated in the non-linear region. This pre-distortion works by means of a negative feedback loop in which, in contrast to the Cartesian loop, not the IQ components of the output signal are controlled, but the amount and the phase of said output signal. An envelope detector detects the amplitude of the signal to be amplified, said amplitude being continuously compared, with regard to its feedback, with the envelope of the output signal of the power amplifier, the comparison result being applied to a variable attenuator which attenuates the input signal before the power amplifier in a suitable manner so as to produce an output signal which is as linear as possible. The phase pre-distortion is carried out by means of a phase locked loop receiving the signal to be amplified as an input signal. A part of the output signal of the amplifier is also inputted in the phase locked loop by means of a mixer, a local oscillator and a phase shift circuit, said phase locked loop supplying a local oscillator signal for a mixer preceding the power amplifier in order to suitably pre-distort the phase of the signal to be amplified. This circuit operates in a fully analogous manner and is based on a substantially continuous feedback, if the phase locked loop is locked.
Also U.S. Pat. No. 4,465,980 represents an analog pre-distortion circuit. A detector detects the envelope of a signal to be amplified and applies this signal to a field effect transistor with two gate terminals (xe2x80x9cdual gate FETxe2x80x9d). The signal to be amplified is applied to the other gate of the dual gate FET. The radio frequency input signal is suitably pre-distorted by controlling the operating point of this dual gate FET in a suitable manner so as to compensate the non-linear amplification of a power amplifier connected to the drain terminal of said dual gate FET via a matching network.
DE 3312030 A1 discloses an amplifier with pre-distortion compensation, which uses pre-distortion components produced by a power amplifying element, which is similar to the amplifying element, so as to achieve an effective linearization for substantially reducing all intermodulation distortion products. Furthermore, an additional feedback circuit can be provided so as to achieve a further reduction of non-linearities.
GB 8723874 discloses a linearity correction circuit operating in an intermediate frequency region so as to introduce a suitable pre-distortion in an amplitude envelope for compensating the non-linearity of the power amplifier stages. An array of parallel current sources, each of which is adapted to be adjusted in response to a predistortion over a respective amplitude band, feeds a current which is sufficient for introducing a suitable differential voltage at the output. In this circuit, a phase pre-distortion does apparently not take place.
EP 0 658 975 A1 refers to a baseband pre-distortion system for the adaptive linearization of power amplifiers and to a radio transmitter making use of the pre-distortion system. In so doing, two error tables, viz. one for the amplitude and one for the phase, are updated, the content of said tables being used for correcting the baseband sampling values. The content of the tables is obtained by accumulating a suitable weighted difference between sampling values which are inputted in the pre-distortion unit and a demodulated feedback value.
EP 0 608 697 A1 discloses a modulation method and a modulation circuit for radio frequency signals. In the case of this modulation method and modulation circuit, the LF modulation signals, which are modulated upon an HF carrier by means of a complex IQ modulation, are pre-distorted in the LF region in such a way that tolerances of the HF components of the modulator, i.e. of the 0xc3x8 to 90xc3x8 hybrid and of the mixers, which cause unbalances between the I and the Q branch, are compensated for. The predistortion of the LF modulation signals guarantees that e.g. in the case of a single-sideband modulation the carrier and the undesired sideband are sufficiently suppressed.
The technical publication IEE Proceedings: Communications, Vol. 143, No. 2, Apr. 1, 1996, pp. 78-86, Ghaderis M. et al, xe2x80x9cFast Adaptive Polynomial I and Q Pre-distorter with Global Optimizationxe2x80x9d, with regard to which the independent claims have been limited, discloses a linearization structure for a radio frequency amplifier, the pre-distortion being implemented on the intermediate frequency. For this purpose, the envelope of an intermediate frequency signal is detected and quantized in the case of a digital implementation. Furthermore, a lookup table for implementing amplification functions is addressed, an analog implementation being, however, preferred to make things easier. These amplification functions represent the inverse non-linearities of the radio frequency power amplifier whose non-linearities are approximated by means of polynomial functions. An IQ modulator causes a pre-distortion of the LF signal, the IQ modulation signals depending on the amplification functions and the envelope of the IF signal.
DE 19 631 388 A1 relates to a radio frequency pre-distortion system for a non-linear channel, wherein the envelope of a signal to be transmitted via a non-linear channel is detected and wherein quantized envelope values are generated. Complex pre-distortion coefficients are stored in a table, the complex pre-distortion coefficients depend on the quantized envelope values and on the transmission function of the non-linear channel which is determined in advance. An IQ modulator is provided for modulating the signal to be transmitted using the complex pre-distortion coefficients before the signal is transmitted via the non-linear channel. The modulation is performed such that the distortion introduced by the non-linear channel is substantially compensated according to magnitude and phase. The above system using a pre-distortion in the radio frequency range rarely provides an adequate compensation because of interference signal portions in channels immediately adjacent to the usable channel. Thus, high demands are made on the bandpass filter connected to the output of the non-linear channel, i.e., the non-linear power amplifier.
U.S. Pat. No. 5,049,832 describes a system for amplifier linearization by adaptive pre-distortion. According to U.S. Pat. No. 5,049,832, squared magnitude values of a complex base-band signal are derived. Based on the squared magnitude values, a look-up table is accessed. Each table entry of the look-up table corresponds to a squared magnitude value. The table entries for respective squared magnitude values are set to provide pre-distortion for the input signal which compensates for level-dependent non-linearities of the gain of the amplifier. For any input power, the optimum value of the complex gain of the pre-distortion, i.e., the table entries, is determined by equating the composite pre-distorter/power amplifier non-linearity to a nominal constant amplitude gain of the power amplifier. The complex base-band signal is pre-distorted by the table entries, and the pre-distorted input signal is applied to the power amplifier. The pre-distorted input signal is obtained using complex multiplication of the complex input signal by the complex table entries. The system described in U.S. Pat. No. 5,049,832 suffers from the disadvantage that the linearization gain, i.e., the reduction of unwanted spurious emission in the channels immediately adjacent to the useful channel, will strongly decrease if the crest factor of the RF signal exceeds the back-off of the dynamic range of the amplifier, i.e., the reserve of the dynamic range of the amplifier. The crest factor of OFDM signals normally exceeds the back-off of the dynamic range of the power amplifier used in the output stage of a digital broadcast transmitter.
It is the object of the present invention to provide an apparatus and a method for further reducing spurious emission in channels adjacent to a useful channel of a radio frequency final stage.
According to a first aspect of the invention, this object is achieved by a circuit for reducing adjacent-channel interference by pre-linearizing and pre-distorting an input signal to be transmitted via a power amplifier having a non-linear transmission characteristic and a limited dynamic range, wherein the circuit has the following features:
a pre-linearization signal generation unit for producing a pre-linearization signal reflecting a signal portion of said input signal exceeding the dynamic range of said power amplifier;
an element for expanding said pre-linearization signal along the time axis to produce an expanded pre-linearization signal reflecting an expanded version of said signal portion of said input signal exceeding said limitation;
an element for combining said expanded pre-linearization signal and said input signal, such that said expanded version of said signal portion of said input signal exceeding said limitation is subtracted from said input signal, to produce a pre-linearized signal;
a pre-distorting unit for pre-distorting said pre-linearized signal using complex pre-distortion coefficients which depend on the power of said input signal or the power of said pre-linearized signal and the non-linear transmission characteristic of said power amplifier determined in advance, such that the distortion introduced by the non-linear transmission characteristic of said power amplifier is substantially compensated for according to magnitude and phase.
According to a second aspect of the invention, the above object is achieved by a method for reducing adjacent-channel interference by pre-linearizing and pre-distorting an input signal to be transmitted via a power amplifier having a non-linear transmission characteristic and a limited dynamic range, comprising the steps of:
producing a pre-linearization signal reflecting a signal portion of said input signal exceeding the dynamic range of said power amplifier;
expanding said pre-linearization signal along the time axis to produce an expanded pre-linearization signal reflecting an expanded version of said signal portion of said input signal exceeding said limitation;
combining said expanded pre-linearization signal and said input signal, such that said expanded version of said signal portion of said input signal exceeding said limitation is subtracted from said input signal, to produce a pre-linearized signal;
pre-distorting said pre-linearized signal using complex pre-distortion coefficients which depend on the power of said input signal or the power of said pre-linearized signal and the non-linear transmission characteristic of said power amplifier determined in advance, such that the distortion introduced by the non-linear transmission characteristic of the power amplifier is substantially compensated for according to magnitude and phase.
According to the present invention, a pre-distorting unit, preferably comprising an estimator, a table containing complex pre-distortion coefficients and a pre-distorter is used along with a pre-linearizing unit using a time-dispersive element (for producing a pre-linearization signal expanded in the in direction of the time axis) in order to conduct a pre-treatment of a signal to be transmitted via a power amplifier of an RF output stage. The pre-distorting unit is not able to handle spurious emission in channels adjacent to the useful channel due to cutting-off of signal portions of the input signal exceeding the dynamic range of the power amplifier by the power amplifier. In order to reduce such spurious emission mentioned above, the time dispersive linearization means for producing the pre-linearized signal is provided according to the present invention.
The inventors recognized that it is not sufficient to produce a pre-linearized signal by limiting the input signal to an upper limitation corresponding to a boundary of the dynamic range of the power amplifier, since purely cutting-off single portions exceeding the upper limitation would produce sharp edges in the input signal, such that spurious adjacent-channel emission in the output of the power amplifier cannot be satisfactory obtained. Thus, according to the present invention, an element for expanding the pre-linearization signal reflecting that signal portion of the input signal exceeding the limitation is provided. Such element is preferably a bandpass filter or a lowpass filter. By expanding the pre-linearization signal to be subtracted from the input signal along the time axis, interference signal portions in the frequency spectrum of the output of the power amplifier adjacent to the useful channel are shifted into a spectral range spaced from the frequency range of the useful signal, i.e., the useful channel.
With respect to a pre-linearization using a time dispersive element, reference is made to the non-prepublished German patent application 19927952.7-35 which goes back to the applicant of the present application. DE 199 27 952.7 relates to a system for conducting a pre-linearization of an input signal to be transmitted via a power amplifier of an RF output stage wherein an estimator produces a fault signal by estimating a fault caused by the non-linearity of the power amplifier. The fault signal is expanded in the direction of the time axis in order to produce a correction signal. The correction signal is subtracted from the signal to be transmitted. By expanding the fault signal in the direction of the time axis, the interference signal portion due to the non-linearity of the power amplifier is shifted away from the useful frequency range in the frequency spectrum of the output signal of the non-linear power amplifier. However, it has been found that such a pre-linearization using a time dispersive element, i.e., an element which expands a signal into the direction of the time axis, will not provide satisfactory results if the input signal of an amplifier having high phase distortions, travelling-wave tubes for example, should be pre-linearized. In such a case, the linearization gain of the linearization means using a time dispersive element is substantially reduced.
The present application is based on a combination of a pre-distorting unit, such as described in U.S. Pat. No. 5,049,832, for example, and a pre-linearizing unit using a time dispersive element. The combination of power amplifier and pre-distorting unit represent a so-called xe2x80x9csoft limiterxe2x80x9d, i.e., an amplifier having a constant gain up to the boundary of the dynamic range and a hard limitation beyond the upper boundary of the dynamic range. Thus, the limiter used in the inventive circuit for reducing adjacent-channel interference and the combining element for producing the pre-linearization signal reflecting a signal portion of the input signal exceeding the limitation represent an estimator for producing a fault signal being an estimation of the fault produced by the power amplifier after non-linearities inside the dynamic range thereof have been addressed by the pre-distorting unit.
In preferred embodiments of the present invention, the limiter of the inventive circuit comprises an estimator for determining an estimation signal based on the power of the input signal, a table for storing real coefficients, wherein the real coefficients depend on the estimation signal and the upper boundary of the dynamic range of the power amplifier, and an element for applying the real coefficients to the input signal such that the limited input signal is produced. Thus, the limiter imitates the performance of the power amplifier in that it estimates the output signal of the power amplifier on the basis of the input signal and real coefficients stored in a table. The real coefficients inside the dynamic range of the power amplifier are constant greater than zero, and are preferably one. Beyond the dynamic range of the power amplifier, the real coefficients are adjusted such that the magnitude of the output of the element for applying the real coefficient to the input signal, preferably a multiplier, is held on a constant value, irrespective of the input signal.
It is not necessary to have real coefficients and imaginary coefficients in the table of the limiter, since phase distortions are addressed by the pre-distorting unit, rather than by the pre-linearizing unit.
In a first embodiment of the present invention, the pre-linearizing unit, i.e., the unit for producing a pre-linearized signal using a time dispersive element and the pre-distorting unit are connected in series, wherein the output of the pre-linearizing unit is connected to the input to the pre-distorting unit.
In a second embodiment of the present invention, the pre-linearizing unit and the pre-distorting unit are xe2x80x9cinterleavedxe2x80x9d wherein the pre-linearizing unit and the pre-distorting unit use the same input signal such that the same estimator can be shared by the pre-distorting unit and the pre-linearizing unit. The second embodiment accepts an approximation with attendant fault. However, it provides a satisfactory reduction of adjacent-channel interference using a time dispersive linearizing unit even in the case of highly non-linear amplifiers. According to the second embodiment, a single estimator is required, contrary to two estimators in case of a simple series connection. Therefore, four multipliers will be saved if a means for deriving the sum of squares of the absolute value of the real and the imaginary parts of the input signal is used as the estimator. Thus, the second embodiment provides an inexpensive solution when compared with a simple series connection of the pre-linearizing unit and the pre-distorting unit.
The inventive circuit for reducing adjacent channel interference is preferably adaptive in that means are provided in order to adapt the coefficients in the tables of the pre-distorting unit and/or the pre-linearizing unit to take changes of the transmission characteristics of the power amplifier into account. Such changes of the transmission characteristics of the power amplifier can take place because of ageing, temperature changes, and so on, for example.
As mentioned above, the time dispersive element is preferably a time dispersive bandpass filter or lowpass filter which reshapes short interference pulses in longer interference pulses in order to effect an expansion of the pre-linearization signal in the direction of the time axis. Filter coefficients of the bandpass filter and the lowpass filter, respectively are preferably determined based on the transmission characteristic of the non-linear power amplifier. To this end, the spectral power of the output signal of the power amplifier can be measured and the power in the channels adjacent to the useful channel can be used as criteria for optimizing the filter coefficients. For such an optimization, known algorithms can be used, the simplex algorithm, for example. It should be clear that any time dispersive element effecting an expansion of the estimated pre-linearization signal can be used apart from the bandpass filter and the lowpass filter mentioned above.
In particular, the present invention is useful for a pre-treatment of signals which have been subjected to a multi-carrier modulation. However, the present invention can be used in connection with other signals comprising high crest factors, CDMA signals, for example.
The present invention is preferably performed on a digital input signal in the base-band region. However, the principles of the present invention can be applied to analog input signals in the form of complex envelopes as well. Moreover, the principles of the present invention also apply for analog signals in the RF region. Thus, the term xe2x80x9csignalxe2x80x9d used herein is destined to encompass the kinds of signal mentioned above wherein respective adaption of the methods and apparatus described with respect to the preferred embodiments of the present invention are obvious for those skilled in the art.
Therefore, the present invention provides apparatus and method which will permit improved reduction of adjacent channel interference even in case of RF signals having a crest factor which exceeds the back-off of the dynamic range of the power amplifier and even in case of power amplifiers having high phase distortions, travelling-wave tubes, for example. According to the present invention, interference signal portions at the output of the power amplifier are as low as possible. Moreover, interference signal portions in the output signal of the power amplifier are shifted away from channels adjacent to the useful channel toward channels spaced from the useful channel.